When a carbon fiber strand is produced from an acrylic fiber strand, it is necessary to carry out a preoxidizing treatment (flame resistance providing treatment) in an atmosphere of an oxidizing gas at a temperature generally of from 200.degree. to 300.degree. C. and then a carbonization treatment or graphitization treatment in an inert gas atmosphere at a temperature of 350.degree. C. or higher. Methods for production of carbon fibers are given in detail in, for example, U.S. Pat. Nos. 4,069,297, 4,073,870 and 4,321,446. Especially, during the pre-oxidizing treatment at 200.degree. to 300.degree. C., it is important to prevent the mutual coalescence of filaments which constitute the fiber strand. For this purpose, methods which involve applying various silicone oils to the acrylic fiber strand have been proposed, such as the use of aminopolysiloxane oils as disclosed in JP-A-52-24136 and JP-A-61-167024. (The term "JP-A" as used herein means an "unexamined published Japanese patent application".)
Although the aminopolysiloxane oils applied to water-swollen filaments after spinning in the production of acrylic fiber strands are effective for the purpose of preventing coalescence during a preoxidation treatment, the surface of the fibers becomes water repellent which subsequently causes the generation of static electricity in dried fibers and the deposition of viscous scum on rollers and guides. Because of this, bundlability of the filaments in a strand is disturbed which results in problems such as fluffing, winding of filaments and the like.
In order to overcome these problems, various methods have been proposed, such as a method in which a copolymer containing both amino and polyoxyalkylene groups in one molecule is applied to the fiber strand (JP-A-61-97477 (corresponding to U.S. Pat. No. 4,830,845)), and a method in which a mixture of an aminopolysiloxane and various additives is applied to the fiber strand (JP-A-56-49022 (corresponding to U.S. Pat. No. 4,378,343), JP-A-55-103313 (corresponding to U.S. Pat. No. 4,259,307), JP-A-2-91224, JP-A-2-91225 and JP-A-2-91226).
Since in such methods some antioxidizing agents are used, film formation of the aminopolysiloxane is delayed or inhibited. Therefore, spreading of fiber strands which is caused by the wind pressure of a circulating oxidizing gas in a normal preoxidizing step cannot be prevented. In addition, because of the addition of antioxidizing agents, thermal decomposition of the aminopolysiloxane shifts from the preoxidizing step to the subsequent carbonization step which is operated at a higher temperature. The gas generated during the carbonization step not only causes corrosion of the surface of carbon fibers but also causes difficulty in achieving long term continuous operation due to deposition of the gas on the furnace wall.